1. Field of the Invention
The present invention relates to an information processing system comprising a computer system having a CPU for performing a normal arithmetic process and a memory, and an input/output (I/O) device having a function of performing an I/O process in response to an I/O request received from the computer system, and informing end of the I/O process to the computer system.
2. Related Background Art
Conventionally, in a system of this type, an I/O device receives an I/O request from a program operating on a computer system. When an I/O process ends, the I/O device issues an interruption signal to the computer system or changes the value of a register, which can be read out by the computer system, thereby informing the end of the I/O process to the computer system. Therefore, the program operating on the computer system issues an I/O request after the entry of a program to be executed after the end of an I/O process is registered in advance in a corresponding interruption vector, or senses a register in the I/O device to wait for the end of the I/O process after it issues an I/O request, and thereafter, starts a program to be executed after the end of the I/O process. In this case, the former I/O processing system is called an interruption system, and the latter I/O processing system is called a polling system.
However, in the prior art, control of the I/O device must be programmed by permanently selecting one of the interruption system and the polling system. When the interruption system is selected, a CPU can execute another job until the end of I/O. However, when an interruption process is started, the CPU status (register contents) must be saved and returned. For this reason, when the I/O processing time (from an I/O request to I/O end) executed by the I/O device is short, the processing efficiency is undesirably lowered.
On the other hand, when the polling system is selected, if the I/O processing time is short, the processing efficiency is better than that of the interruption system since the polling system requires neither of the CPU status save and return processes. However, if the I/O processing time is long, since the CPU must keep polling during this time, the throughput of the entire system is lowered. That is, when an I/O device such as a hard disk device for which the processing time changes depending on, e.g., the current head position to be controlled, an optimal process cannot always be performed even if such control is programmed using either system.